Two new rare-earth oxyborates Ba4BiTbO(BO3)4 and Ba1.54Sr2.46BiTbO(BO3)4 and luminescence properties of the Ba4BiTb1−xEuxO(BO3)4 phosphors

Single crystals of two new terbium oxyborates Ba4BiTbO(BO3)4 and Ba1.54Sr2.46BiTbO(BO3)4 were obtained by the high-temperature solution method. They crystallize in the hexagonal P63/mmc group (Z = 2) with lattice parameters of a = 5.41865(9) Å, c = 26.3535(5) Å, V = 670.12(3) Å3 and a = 5.36534(19) Å, c = 26.0661(10) Å, V = 649.83(5) Å3, respectively. Their crystal structures feature two kinds of layers: [Tb(BO3)2]n3n− formed by corner-sharing TbO6 octahedra and BO3 triangles, as well as [Bi(BO3)2O]n5n− consisting of Bi2O13 dimers and BO3 groups, with alkali-earth cations sitting inside and between the layers. In addition, solid solutions of Ba4BiTb1−xEuxO(BO3)4 (0 ≤ x ≤ 0.2) were prepared via the solid-state reaction method. The obtained products were characterized by powder XRD, SEM, IR/Raman, XPS, DRS, and luminescence spectroscopy. It was found that as the Eu3+ doped content varies from x = 0 to 0.2, the emission color of the Ba4BiTb1−xEuxO(BO3)4 phosphors can be adjusted from cyan to near-white and then to orange-red or from green to orange and then to red under the excitation of 349 and 377 nm, respectively. Furthermore, the emission intensities and chromaticity coordinates were found to be sensitive to the temperature for the phosphor Ba4BiTb0.999Eu0.001O(BO3)4 upon 377 nm excitation. The above results demonstrate that Ba4BiTb1−xEuxO(BO3)4 phosphors have potential as multifunctional materials for solid-state lighting and temperature sensing applications.


Introduction
Among solid-state lighting technology, white light-emitting diodes (w-LEDs) are excellent candidates to replace conventional incandescent lamps for their merits of high brightness, low energy consumption, long lifetime, small size, environmental-friendly nature, and so on. 1,2In the past years, cadmium chalcogenide quantum dots (QDs) have been proposed to be the promising alternatives to the phosphors in w-LEDs due to their tunable and narrow-band emission, high photoluminescence quantum yield (PLQY), and good stability. 3ttenuation situations of each phosphor will result in an unstable white light. 12Therefore, it has also been proposed to manufacture w-LEDs by coating single-phase white-lightemitting phosphors on UV/n-UV LED chips, which has always been a research hotspot in solid-state lighting.
6][27] The syntheses of these phosphors require a reducing atmosphere, as the rareearth ions Ce 3+ and Eu 2+ are in their lower valence states.As everyone knows, Bi 3+ ion is a representative non-rare-earth activator with 6s 2 electronic conguration, which can exhibit blue emission generated by the parity-allowed 6s 2 -6s6p inter-congurational transitions. 28Bi 3+ has low toxicity and is stable in the air, and its synthesis condition is relatively mild compared to Ce 3+ and Eu 2+ .0][31][32] However, doping with three ions such as Bi 3+ , Tb 3+ , and Eu 3+ can lead to complicated energy transfer, so tuning to accurate white color remains still a challenge.
In the previous study, we systematically investigated the BaO-Bi 2 O 3 -PbO-Eu 2 O 3 -B 2 O 3 system and discovered a new borate Ba 3 BiPbEuO(BO 3 ) 4 . 33On this basis, we synthesized a series of Ba 3 BiPbY 1−x Eu x O(BO 3 ) 4 (0 # x # 1) solid solutions and studied their luminescence properties.However, there have been no reports of doping Tb 3+ activators in this host to obtain white or color-adjustable phosphors, and of course, temperature sensing properties of this type of phosphor have also not been investigated so far.Because Pb belongs to a toxic element that limits the practical application of this phosphor in w-LEDs.Our attempt to replace Pb 2+ with iso-valent Ba 2+ and Sr 2+ ions led to the characterization of two new rare-earth oxyborates, Ba 4 BiTbO(BO 3 ) 4 and Ba 1.54 Sr 2.46 BiTbO(BO 3 ) 4 .In this work, we rst introduced the syntheses and crystal structures of these two new phases, and then, described the luminescence performance, energy transfer process, and temperature sensing behavior of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 (0 # x # 0.2) uorescent powders.It was found that the syntheses of this series of new borates can be carried out at a relatively low temperature without the need for a reducing atmosphere.Besides, the white light and color-tunable emission from green to red can be realized in Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 by simply changing the concentrations ratio of Tb 3+ /Eu 3+ and selecting appropriate excitation irradiation.Additionally, a strong temperature sensitization phenomenon was found for this phosphor under 377 nm excitation, suggesting that Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 is a kind of luminescent material with potential application value in the eld of w-LEDs and temperature sensors.
Note that Ba 4 BiTbO(BO 3 ) 4 and Ba 1.54 Sr 2.46 BiTbO(BO 3 ) 4 are the rst quaternary (quinary) borate found in the BaO (and SrO)-Bi 2 O 3 -Tb 2 O 3 -B 2 O 3 system, and also, the luminescence properties of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 (0 # x # 0.2) are presented here for the rst time.This study also reveals that Ba 4 BiLnO(BO 3 ) 4 (Ln = trivalent rare-earth ions) forms a large family of compounds, in which Ba 2+ can be partially replaced by other divalent cations, such as Pb 2+ and Sr 2+ .We believe that this work will enrich the structural chemistry of borates and help the development of new emission-tunable phosphors for w-LED applications and highly efficient luminescent thermometers for temperature-sensing applications.

Materials and methods
All chemicals for the synthesis were purchased from Sinopharm Chemical Reagent Co. Ltd, including BaCO 3 (A.R.), SrCO 3 (A.R.), Bi 2 O 3 (A.R.), Y 2 O 3 (99.99%),Tb 4 O 7 (99.99%),Eu 2 O 3 (99.99%)and H 3 BO 3 (A.R.).Powder X-ray diffraction (XRD) measurements were conducted on a Bruker D8 ADVANCE diffractometer, which operated at 40 kV and 40 mA with Cu K a radiation (l = 1.5406Å).Scanning electron microscope (SEM) images were taken using a Hitachi SU8020 instrument (accelerating voltage 20 kV) equipped with an energy-dispersive X-ray detector (EDX).Infrared (IR) spectra were obtained with a VERTEX 70 Fourier Transform Infrared Spectrometer (Bruker).Raman spectroscopy data were recorded on the powder sample by a Renishaw InVia Raman spectrometer, which has a spectral resolution of 1 cm −1 and is equipped with a confocal DM 2500 Leica optical microscope (with a 50× objective) and a 785 nm diode laser.Diffuse Reectance Spectra (DRS) were measured with a Hitachi UH4150 UV-visible/NIR spectrophotometer, scanning at 120 nm min −1 .The elemental valence states were analyzed via X-ray Photoelectron Spectroscopy (XPS) on a Thermo Scientic ESCALAB 250Xi spectrometer with monochromatized Al K a radiation (hn = 1486.6eV).Photoluminescence excitation (PLE) and emission (PL) spectra as well as luminescence decay curves were collected via the Edinburgh FLS 1000 uorescence spectrometer.A 150 W Xe lamp and a 60 W mF ash lamp were used as the excitation source for the steady-state spectrum and decay curve measurements, respectively.Quantum yield (QY) and variable temperature PL spectra were measured by employing a barium sulfate-coated integrating sphere (150 mm in diameter) and a temperature controller (Oxford, OptistatDN2) attached to the FLS 1000 system, respectively.

Synthetic procedures
Single crystals of the title compounds were grown via the hightemperature solution method.Typically, a mixture of 0.2894 g BaCO 3 , 0.9112 g Bi 2 O 3 , 0.1828 g Tb 4 O 7 and 0.0907 g H 3 BO 3 (the molar ratio of 6 : 8 : 1 : 6) for Ba 4 BiTbO(BO 3 ) 4 , and a mixture of 0.3879 g BaCO 3 , 0.2031 g SrCO 3 , 0.6411 g Bi 2 O 3 , 0.0735 g Tb 4 O 7 and 0.1945 g H 3 BO 3 (the molar ratio of 20 : 14 : 14 : 1 : 32) for Ba 1.54 Sr 2.46 BiTbO(BO 3 ) 4 were transferred to Pt crucibles aer thorough grinding, respectively.In the furnace, these mixtures were gradually heated to 950 °C, where they were kept for 6 h, then cooled to 700 °C at a rate of 1.5 °C h −1 , and further to 400 °C at 5 °C h −1 , followed by cooling to room temperature at 20 °C h −1 .The colorless transparent crystals of the title compounds were observed, which showed a hexagonal platelike habit with various sizes ranging from 0.1 to 1.0 mm in diameter and several tenths of a millimeter in thickness.They were mechanically isolated from the solidied melt and further examined through SEM-EDX analyses, which conrmed the existence of heavy elements of Ba, Bi, Tb, and O (Ba, Sr, Bi, Tb, and O) (B is too light to be detected, see Fig. S1 and S2 †).
Polycrystalline samples of Ba 4 BiYO(BO 3 ) 4 and Ba 4 BiTb 1−x -Eu x O(BO 3 ) 4 (x = 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.2, and 1) were prepared by the traditional solid-state reaction method.The required metal carbonates or oxides and boric acid were weighted in stoichiometric ratio and thoroughly mixed and ground in an agate mortar for 30 min.Subsequently, they were transferred into alumina crucibles and preheated at 500 °C for 12 h to remove CO 2 and H 2 O. Then further sintering at 830 °C for 96 h with multiple intermittent grinding was implemented to ensure the complete chemical reaction.Finally, the obtained products were naturally cooled to room temperature and ground again for further characterization.

Single-crystal X-ray diffraction
Single-crystal XRD data of two crystals were collected at room temperature on a SuperNova (Mo) X-ray source (l = 0.71073 Å).Data reduction, cell renements, and analytical absorption corrections were accomplished using CrysAlisPro soware. 34he structures were solved by direct methods and rened with anisotropic displacement parameters for all atoms by fullmatrix least-squares tting on F 2 using SHELX-2019. 35or Ba 4 BiTbO(BO 3 ) 4 , the site-occupancy renements indicated that there is an atomic site (Wyckoff 4e position) that was co-occupied by Ba and Bi atoms.In the initial renement, Ba 2+ and Bi 3+ cations were placed at the same 4e site, with their atomic coordinates and anisotropic displacement parameters being constrained to be identical.Aer convergence of the renement, rather large displacement parameters were obtained for this (Ba/Bi) site, suggesting additional positional disorder.Eventually, a signicant improvement in the renement was achieved aer splitting the (Ba/Bi) into two close positions.A similar situation was found in Ba 1.54 Sr 2.46 -BiTbO(BO 3 ) 4 , except that (Ba/Bi) was replaced by (Sr/Bi).The structures were checked with the program MISSYM, and no obvious additional crystallographic symmetry was detected. 36eneral crystallographic information is presented in Table 1.
Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters, as well as selected bond lengths and angles, are provided in Tables S1-S3.†

Crystal structure description
Since Ba 4 BiTbO(BO 3 ) 4 and Ba 1.54 Sr 2.46 BiTbO(BO 3 ) 4 are isostructural, we chose Ba 4 BiTbO(BO 3 ) 4 to describe the crystal structure.The fundamental building blocks in this structure are TbO 6 octahedra, BO 3 triangles, and Bi 2 O 13 dimers composed of two corner-sharing BiO 7 hexagonal pyramids, as shown in Fig. 1(a).Within the ab plane, TbO 6 octahedra are arranged hexagonally and further bridged by B1-centered BO 3 groups via common O atoms to form a 2D innite sheet of [Tb(BO 3 ) 2 ] n 3n− [layer A, see Fig. 1(b)].There is another type of 2D sheet that has a composition [Bi(BO 3 ) 2 O] n 5n− [layer B, see Fig. 1(c)].This layer is constructed by corner-sharing Bi 2 O 13 dimers, with a pair of boron atoms (two B2) accommodated within one-half of the three-membered rings of the layer.Four sheets, ABA 0 B 0 , are required to complete a translation period of the c-axis direction, where the sheets A 0 and B 0 are related to A and B by a 6 3 -screw axis.Some alkali-earth cations (Ba1 and Ba3) ll the space between the layers, while others (Ba2) reside in the channels that pass through the layers B and B 0 to provide charge compensation.
As shown in Table S1, † there are nine unique atomic sites in the asymmetric unit of Ba 4 BiTbO(BO 3 ) 4 , including 2Ba, 1(Ba/ Bi), 1Tb, 2B, and 3O, all located in crystallographically special positions.Two distinct Ba atoms are both nine-coordinated, but adopt different coordination congurations: Ba1 in an irregular polyhedron and Ba2 in a tri-capped trigonal prism [Fig.1(d)].The Ba-O distances are normal, as shown in Table S2.† Concerning the disordered (Ba/Bi) site, it is found that Ba and Bi atoms are statistically distributed over two very close positions [Ba3-Bi1 = 0.320(3) Å].Each is strongly bonded to one oxygen   6)-1.379(7)Å, and the bond angles deviated slightly from 120°.These are common values as found in other known borates. 33,38The calculated total bond valences for Ba1, Ba2, Tb, B1, and B2 are 2.049, 2.052, 3.282, 2.991, and 2.937, respectively, consistent with their expected oxidation states. 39lthough the title compounds are isostructural with our previously reported Pb 2+ -analog, Ba 3 BiPbEuO(BO 3 ) 4 , their crystal structures show a certain difference. 33For example, in Ba 3 BiPbEuO(BO 3 ) 4 , both Pb 2+ and Bi 3+ cations are located at the same 4e site with identical positional and thermal parameters, while in Ba 4 BiTbO(BO 3 ) 4 [Ba 1.54 Sr 2.46 BiTbO(BO 3 ) 4 ], this site is split into two very close positions, occupied by Ba 2+ and Bi 3+ (Sr 2+ and Bi 3+ ), respectively.This is understandable because both Pb 2+ and Bi 3+ have a 6s 2 electron conguration and, of course, a similar coordination environment, while the ionic radii and coordination geometries around Ba 2+ and Bi 3+ as well as Sr 2+ and Bi 3+ are very different.Inspection of structural details (Tables S1-S3 †) reveals that when Sr 2+ partially replaces Ba 2+ in Ba 4 BiTbO(BO 3 ) 4 to form Ba 1.54 Sr 2.46 BiTbO(BO 3 ) 4 , the Ba1 and Ba2 sites are mixed with some Sr atoms, with the rened compositions Ba 0.58(2) Sr 0.42 (2) and Ba 0.38(3) Sr 0.62 (3) , respectively.The Ba3/Bi1 site is still statistically distributed, but Ba3 is completely replaced by Sr3.Given identical coordinated numbers, (Ba/Sr)-O distances are signicantly smaller than the corresponding Ba-O ones as expected.In addition, there is also a remarkable reduction in cell axis lengths and volume due to the smaller size of Sr 2+ compared with Ba 2+ (Table 1).What's more, this work also indicates that Ba 4 BiLnO(BO 3 ) 4 (Ln = trivalent rare-earth ions) forms a large class of compounds, where Ba 2+ can be partially substituted by other divalent ions, like Pb 2+ and Sr 2+ .

Phase purity and morphology
A series of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 (0 # x # 1) samples were prepared and their crystallinity and phase purity were assessed by powder XRD, as shown in Fig. 2(a).For comparison, the simulated pattern of Ba 4 BiTbO(BO 3 ) 4 based on its single-crystal structure is provided at the bottom of the gure.All observed XRD patterns are similar to each other and also consistent with the simulated pattern of the fully Tb 3+ sample, showing that the ), but smaller than that of Bi 3+ (r = 1.07 Å, CN = 7), 40,41 such a shi implies that Eu 3+ ions prefer to substitute Tb 3+ instead of Bi 3+ sites in the Ba 4 BiTbO(BO 3 ) 4 host.This is understandable because the coordination conguration around Eu 3+ and Tb 3+ is very similar and relatively regular, while that around Bi 3+ is severely distorted due to the steric effect of 6s 2 lone pair electrons [see Fig. 1(d)].
To further recognize the effect of doping Eu 3+ ions on the crystal structures of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 , Rietveld renements were conducted on the powder XRD proles of Ba 4 -BiTb 1−x Eu x O(BO 3 ) 4 (x = 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.2, and 1) using TOPAS Academic soware. 42The initial model used for the Rietveld renements was the single-crystal structure of Ba 4 BiTbO(BO 3 ) 4 (Tables 1 and S1 †).The constraint condition is that atomic coordinates of B and O, all atomic occupancies, and isotropic displacement parameters were xed, while the positional parameters of heavy atomic sites [including z(Ba1), z(Ba3), and z(Bi1)] and lattice constants (a and c) were rened along with other parameters.The nal renement results are presented in detail in Table S4 † as well as Fig. 2(b), (c) and S3.† As representatives, the lattice parameters of the species for 0 # x # 0.2 are plotted against the substitution level x in Fig. 2(d).The renements proceeded smoothly and the residual factors R wp are all less than 10.4%, which further conrmed that this series of solid solutions are isomorphic and crystallize in the hexagonal system with space group P6 3 /mmc.In addition, the cell parameters (a, c, and V) show an obvious linear expansion along with the increase in Eu 3+ concentration.These results conform to Vegard's rule, indicating that Eu 3+ ions are completely dissolved in the Ba 4 BiTbO(BO 3 ) 4 host to form perfect solid solutions of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 .
Because particle size and surface roughness are important factors inuencing the luminescence performance of uorescent powders, FE-SEM and EDX analyses were conducted to investigate the morphological characteristics of the obtained powder samples.Fig. 3 displays the FE-SEM images, EDX spectrum, and elemental mapping of the prepared Ba 4 -BiTb 0.999 Eu 0.001 O(BO 3 ) 4 phosphor.It can be seen that the particles are aggregated together and have good crystallinity with smooth surface and irregular morphology, and the particle size is in the range of several micrometers.This may be due to the sample being obtained through a solid-state reaction route of intermediate grinding.The EDX pattern conrms the presence of the dominant elements Ba, Bi, Tb, Eu, B, and O, all homogeneously distributed over the same selection region of the sample, as demonstrated in the elemental mapping images.This indicates that the target solid solution has been successfully synthesized (note that the boron content cannot be accurately measured due to its low atomic weight, while the gold signal comes from the gold sputtering process).

IR and Raman spectra
Although single crystal XRD analyses indicated the presence of BO 3 groups in the title compounds, IR and Raman spectroscopy were still studied to further conrm the coordination environment of B atoms.Ba 4 BiLnO(BO 3 ) 4 (Ln = trivalent rare-earth ions) crystallizes in the centrosymmetric P6 3 /mmc (D 6h 4 , No. 194) group (Z = 2).Based on group theoretical analysis, (Ln = Y, Tb and Eu).Three compounds show similar spectral proles, illustrating that they are isostructural to each other, and the introduction of different rare-earth ions does not affect the fundamental vibrational modes of the main structural units.Both IR and Raman spectra above 550 cm −1 can be divided into four regions, of which the area of 1100-1400 cm −1 shows the asymmetric stretching vibrations (n 3 ) of the BO 3 groups.The absorption bands between 850 and 980 cm −1 can be assigned as the BO 3 symmetric stretching vibrations (n 1 ).The absorption caused by the out-of-plane bending (n 2 ) of the trigonal group appears as bands in the range of 700-800 cm −1 .The remaining bands from 550 to 680 cm −1 are the result of the in-plane BO 3 bending (n 4 ).5][46] As everyone knows, for the ideal BO 3 group with D 3h symmetry, the modes n 1 (A 1 0 ) and n 2 (A 2 00 ) should be IR and Raman inactive, respectively. 43The appearance of vibrations n 1 (2A 2u ) in the IR and n 2 (2A 1g ) in the Raman spectra indicates that the BO 3 groups in Ba 4 BiLnO(BO 3 ) 4 deviate from the ideal symmetry.In fact, our structural analysis reveals that there are two crystallographically independent B atoms, both of which occupy the 4f position with local symmetry C 3v instead of D 3h (see Table S1 †).

XPS spectra
To get an idea of the valence states of the constituent elements in the system, we conducted XPS studies on the phosphor Ba 4 BiTb 0.999 Eu 0.001 O(BO 3 ) 4 over a wide energy range, as shown in Fig. S5( The Ba 3d can be deconvoluted into two spinorbital splitting components, Ba 3d 3/2 and Ba 3d 5/2 , located at the binding energies (BEs) of 795.48 and 780.25 eV, respectively. 47The XPS spectrum related to Bi 4f consists of two intense and symmetrical peaks observed at 164.47 (Bi 4f 5/2 ) and 159.25 (Bi 4f 7/2 ) eV with an energy difference of ∼5.22 eV. 48The Tb 3d XPS peaks also exhibit doublet characteristics, which are assigned to Tb 3d 3/2 (∼1278.86eV) and Tb 3d 5/2 (∼1244.18eV), separated by 34.68 eV. 49These BE values can be used as ngerprints to identify the presence of Ba 2+ , Bi 3+ , and Tb 3+ , respectively.In Fig. S5

UV-vis diffuse reection spectra
To investigate the optical absorption properties, the UV-vis diffuse reection spectra of Ba 4 BiLnO(BO 3 ) 4 (Ln = Y, Tb, and Eu) were measured, as shown in Fig. S6.† All samples show a very high reectance between 90 and 100% relative to the white standard BaSO 4 (optical grade) in the visible region, indicating a high optical grade of the as-prepared compounds.Below 400 nm, there is an intense broad absorption band, which is due to the collective effect of the 1 S 0 / 1 P 1 / 3 P 1 transition of Bi 3+ and the host lattice absorption. 28For the Tb and Eu compounds, the Tb 3+ :4f 8 / 4f 7 5d 1 absorption and O 2− / Eu 3+ charge transfer absorption are also located in this area, respectively.Moreover, several sharp characteristic absorption peaks at about 393, 466, 527, 535 and 592 nm are assigned to 7 F 0 / 5 L 6 , 7 F 0 / 5 D 2 , 7 F 0 / 5 D 1 , 7 F 1 / 5 D 1 and 7 F 1 / 5 D 0 transitions of Eu 3+ , respectively, while the peak at 486 nm is ascribed to the 7 F 6 / 5 D 4 transition of Tb 3+ . 54he optical band gap of Ba 4 BiLnO(BO 3 ) 4 can be estimated by the following formula: where hn is the photon energy, A is a proportional constant, E g is the optical band gap energy and the value of n is 1/2 or 2 for an indirect or a direct allowed transition, respectively.The F(R N ) can be described by the Kubelka-Munk function: 56 here, R, K, and S are the reection, absorption, and scattering coefficients, respectively.From the linear extrapolation of the

PLE and PL spectra
As is known to all, Bi 3+ possesses 6s 2 lone pair electrons, therefore, the position of its excitation and emission bands depends largely on the nature of the host lattice.The ground state of Bi 3+ is 1 S 0 and its excited states are composed of 3 P 0 , 3 P 1 , 3 P 2 , and 1 P 1 in order of increasing energy.For the electron transition of Bi 3+ , 1 S 0 / 1 P 1 is parity-and spin-allowed, but it is usually located in the deep ultraviolet region (typically #250 nm), so it is difficult to use in LED devices. 1 S 0 / 3 P 2 is spinforbidden and the luminous intensity is low. 1 S 0 / 3 P 0 is strictly forbidden because the total angular momentum does not change (DJ = 0).In contrast, 1 S 0 / 3 P 1 has the longest luminous wavelength and can even enter the n-UV region (around 350-420 nm).Although it is forbidden according to the spin-selection rule, it will become partially allowed by mixing with singlet and triplet states (DJ = 1). 57he luminescent properties of the title compounds were characterized by the photoluminescence excitation (PLE) and emission (PL) spectra.As depicted in Fig. 4(a), the PLE spectrum of Ba 4 BiYO(BO 3 ) 4 (l em = 397 nm) shows three broad excitation bands, attributing to the 1 S 0 / 1 P 1 transition of Bi 3+ (240 nm), and 1 S 0 / 3 P 1 transitions of Bi 3+ (I) (320 nm) and Bi 3+ (II) (349 nm), respectively, [58][59][60][61] which are related to the crystal structure where the disordered (Ba/Bi) site is split into two positions (Table S1 †).9][60][61] The proles of the emission spectra under 320 and 349 nm excitation are similar to those under 240 nm excitation, except that the emission intensities are much lower.
Fig. 4(c) presents the luminescence spectra of Ba 4 -BiEuO(BO 3 ) 4 .When monitoring 612 nm emission, the PLE spectrum consists of one broad absorption band in the region of 280-357 nm (centered at ∼299 nm) and several sharp excitation peaks in the longer wavelength region.The wide band is the typical absorption of O 2− / Eu 3+ charge transfer superimposed with the 1 S 0 / 3 P 1 transition of Bi 3+ , while the sharp peaks at 362, 382, 394, 414, 465, and 526 nm are attributed to 7 F 0 / 5 D 4 , 5 L 7 , 5 L 6 , 5 D 3 , 5 D 2 and 5 D 1 transitions of Eu 3+ , respectively. 54,62Upon 394 nm excitation, the PL spectrum shows a series of sharp emission peaks within 570-710 nm.These peaks located at 580, 593, 612, 656, and 704 nm are readily assigned to 5 D 0 / 7 F J (J = 0-4) transitions of Eu 3+ , respectively.When excited at 349 nm (Bi 3+ absorption), besides the very small residual emission of Bi 3+ , the strong characteristic emission peaks of Eu 3+ can also be observed, which means an efficient energy transfer from Bi 3+ to Eu 3+ .The comparison of the PLE spectrum for Ba 4 BiLnO(BO 3 ) 4 (Ln = Tb and Eu) and PL spectrum for Ba 4 BiYO(BO 3 ) 4 reveals a signicant spectral overlap between the emission band of Bi 3+ and the f-f absorption of Tb 3+ and Eu 3+ in the wavelength range of 360-425 nm.Therefore, the effective energy transfer from Bi 3+ to both Tb 3+ and Eu 3+ can be expected.It is noticeable that among the observed emission transitions, the magnetic dipole 5 D 0 / 7 F 1 is predominant, suggesting that the surrounding environment of Eu 3+ is centrosymmetric.This is in line with our crystallographic research, which shows that Eu 3+ may replace Tb 3+ and mainly occupies the six-fold coordinated Wyckoff 2a site, which has a regular octahedral coordination conguration.Therefore, the ligand eld around Eu 3+ is centrosymmetric [see Fig. 1(d) and Table S1 †].
Since the UV-excitation at ∼240 nm does not match the output wavelength of commercial n-UV (l em = 350-420 nm) LED chips, and also the Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 (0 # x # 0.2) phosphors show a blue-to-purple emission (without passing through the white light region) under this excitation, we chose 349 nm (Bi 3+ : 1 S 0 / 3 P 1 ) as the excitation wavelength for Bi 3+ to measure the concentration-dependent PL spectra of Ba 4 -BiTb 1−x Eu x O(BO 3 ) 4 .As seen from Fig. 5(b), the spectra exhibit typical Bi 3+ : 3 P 1 / 1 S 0 , Tb 3+ : 5 D 4 / 7 F J (J = 6, 5) and Eu 3+ : 5 D 0 / 7 F J (J = 1-4) transitions in the blue, green, and orange emission regions, respectively.With an increase in the Eu 3+ content (x), the emission intensities of both Bi 3+ and Tb 3+ decline monotonously, whereas that of Eu 3+ rst increases until x = 0.05 and then appears a downfall.This is easy to understand because the occurrence of Bi 3+ / Tb 3+ and Bi 2+ / Eu 3+ energy transfer results in a reduction of Bi 3+ emissions.The incorporation of Eu 3+ into the Ba 4 BiTbO(BO 3 ) 4 host would cause the substantial energy transfer from Tb 3+ to Eu 3+ , and meanwhile, the relatively high efficiency of Bi 3+ -to-Eu 3+ energy transfer would suppress the Bi 3+ -to-Tb 3+ energy transfer, therefore, a decrease in Tb 3+ emissions can be anticipated.In addition, the increase of Eu 3+ emissions with increasing Eu 3+ content up to x = 0.05 is also understandable because both Bi 3+ and Tb 3+ can transfer energy to Eu 3+ .However, once the Eu 3+ content (x) exceeds 0.05, the concentration quenching occurs, resulting in the weakening of the Eu 3+ emission intensity.Moreover, as shown in Fig. 5(c), when excited at 377 nm (Tb 3+ : 7 F 6 / 5 D 3 ), the PL spectra exhibit both Tb 3+ and Eu 3+ emissions, with the latter dominating.As the Eu 3+ concentration increases, the emission intensity of Tb 3+ at 543 nm decreases, while that of Eu 3+ at 593 nm shows the opposite trend, as a consequence of Tb 3+ / Eu 3+ energy transfer.However, if we use 394 nm (Eu 3+ : 7 F 0 / 5 L 6 ) as the excitation wavelength, only the characteristic emissions of Eu 3+ : 5 D 0 / 7 F J (J = 0-4) are visible in Fig. 5(d), indicating that the inverse transition (Eu 3+ / Bi 3+ and Eu 3+ / Tb 3+ ) is forbidden.In this case, the PL intensity increases linearly with the Eu 3+ doping content (x), and no concentration quenching occurs up to x = 0.2.These observations show that the emission proles and color of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 are inuenced not only by the dopant concentration but also by the selected excitation wavelength.[66]

Fluorescence lifetime and energy transfer mechanism
To further illustrate the energy transfer processes, Fig. 6 shows the uorescence decay curves for the Bi 3+ : 3 P 1 / 1 S 0 and Tb 3+ : 5 D 4 / 7 F 5 emission bands of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 under excitation at 349 and 377 nm, respectively.All the decay proles can be tted into a triple-exponential function: in this expression, I 0 and I t represent the PL intensity at time 0 and t, respectively.B 1 , B 2 , and B 3 stand for the tting constants.s 1 , s 2 , and s 3 are lifetimes for the triple-exponential components.The average decay lifetime (s avg ) can be calculated using the equation given below: 67 The s avg values obtained for different phosphors are also provided in Fig. 6, from which it is easy to see that increasing the Eu 3+ concentration causes a reduction in the decay lifetimes of Bi 3+ and Tb 3+ emissions, which strongly supports the energy transfer from Bi 3+ to Tb 3+ /Eu 3+ and from Tb 3+ to Eu 3+ .
Based on the above analyses, a schematic representation of the possible energy transfer pathways in Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 is described in Fig. 7. Upon excitation at 349 nm, the electrons on Bi 3+ ions can be effectively excited from the ground state 1 S 0 to the excited state 3 P 1 .Then they relax nonradiatively to the lowest vibration mode of the 3 P 1 state.Aerward, some of the excited electrons return to the ground state of Bi 3+ to generate the broadband blue emission, while the others can transfer their energy to the 5 D 3 level of Tb 3+ and the 5 L 6 level of Eu 3+ due to energy level matching, followed by non-radiative relaxation from 5 D 3 to 5 D 4 (Tb 3+ ) and from 5 L 6 to 5 D 0 (Eu 3+ ) states.Finally, a series of characteristic emissions of Tb 3+ : 5 D 4 / 7 F 6,5,4,3 and Eu 3+ : 5 D 0 / 7 F 0,1,2,3,4 occur.Apart from the energy transfer from Bi 3+ to both Tb 3+ and Eu 3+ , some of the excited electrons can also transfer their energy from 5 D 3 level of Tb 3+ to the 5 L 6 excited level of Eu 3+ , which enhances the Eu 3+ emission, accompanied by a decrease in the Bi 3+ and Tb 3+ emissions.   When l ex = 377 nm, the chromaticity point can be tuned almost linearly from green (0.2785, 0.5058) to orange (0.4854, 0.4389) and then to red (0.6120, 0.3797) by controlling the concentration ratio of Tb 3+ /Eu 3+ .For an intuitionistic display, digital photographs of the selected phosphors under n-UV irradiation are also presented in Fig. 8, where the near-white emission for Ba 4 BiTb 0.999 Eu 0.001 O(BO 3 ) 4 (l ex = 349 nm) and a variation in the emission color from green to red for Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 (l ex = 377 nm) are visible.Based on the CIE diagrams, it is expected that when the Eu 3+ doping amount is greater than 0.2, the luminescence color of Ba 4 BiTb 1−x Eu x -O(BO 3 ) 4 will change slightly from orange-red to red for l ex = 349 nm.In comparison, it will remain red for l ex = 377 nm, due to the high efficiency of Bi 3+ -to-Eu 3+ and Tb 3+ -to-Eu 3+ energy transfer.What's more, the quantum yields (QYs) of Ba 4 -BiTb 0.999 Eu 0.001 O(BO 3 ) 4 were determined by the integration sphere method to be approximately 0.06% for l ex = 349 nm.The QYs of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 (x = 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.2) for l ex = 377 nm were 0.25%, 0.51%, 0.63%, 1.02%, 1.49%, 3.65%, 4.75%, and 7.06%, respectively.The QYs of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 phosphors (l ex = 377 nm) increase with the increase of Eu 3+ concentration from x = 0 to 0.2, which is attributed to the efficient Tb 3+ / Eu 3+ energy transfer.The low QY values may be because the sample was prepared by a simple high-temperature solid-state reaction, and we believe that the luminous efficiency can be enhanced by optimizing the experimental conditions in future research.

Thermal stability
Thermal stability is an indispensable parameter for novel phosphor material because the LED chips are always operated in a high-temperature environment.The temperaturedependent PL spectra of the phosphor Ba 4 BiTb 0.999 Eu 0.001 -O(BO 3 ) 4 were recorded under two excited wavelengths of 349 and 377 nm, respectively, and the corresponding normalized emission intensities were calculated, as shown in Fig. 9(a) and (b).The PL intensity decreases monotonically with the temperature increasing from 303 to 483 K due to the thermal quenching effect.When the temperature reaches 423 K (150 °C), the PL intensity retains ∼42% (l ex = 349 nm) and ∼17% (l ex = 377 nm) of that at room temperature (303 K), respectively, indicating that the thermal quenching is more obvious for l ex = 377 nm than l ex = 349 nm.To better understand the temperature dependence of photoluminescence, the activation energy where I and I 0 represent the PL intensity at temperatures of T and 303 K, respectively, and A and k are constants (k = 8.62 × 10 −5 eV K −1 ).Fig. 9(c) and (d) show the plots of ln(I 0 /I − 1) against 1/kT for this phosphor under two excitations.The data can be well-tted to straight lines with slopes of −0.156 and −0.281, respectively.Thus, the obtained DE value of 0.281 eV for l ex = 377 nm is greater than that of 0.156 eV for l ex = 349 nm, and both are comparable to those of several previously reported Bi 3+ /Tb 3+ /Eu 3+ co-doped phosphors, such as Ca 2.24 ZrSi 2 -O 9 :0.17Eu 3+ , 0.09Bi 3+ , 0.50Tb 3+ (DE = 0.195 eV), Gd 1.474 MoB 2 -O 9 :0.30Bi 3+ , 0.20Tb 3+ , 0.026Eu 3+ , (DE = 0.17 eV) and LaMoBO 6 :0.4Tb 3+ , 0.005Eu 3+ , 0.02Bi 3+ (DE = 0.19 eV). 30,31,71his phosphor exhibits strong temperature sensitization when excited at 377 nm, and its CIE coordinates shi signicantly from (0.4540, 0.4052) to (0.2891, 0.3308) as the temperature increases from 303 to 483 K, indicating its potential in temperature sensing.Fig. 10 shows the Thus, the sensitivity of Ba 4 BiTb 0.999 Eu 0.001 O(BO 3 ) 4 for temperature sensing is about 0.57% per K. Considering this change in the emission intensity and chromaticity coordinates with temperature, we expect that Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 phosphors excited at 377 nm may be used as luminescent thermometers at this temperature range. 72,73A detailed investigation of this aspect is beyond the scope of this article and will be provided elsewhere.

Conclusions
The investigation of the BaO (and SrO)- and also weakly bonded to six other O atoms to form a hexagonal pyramid, as shown in Fig. 1(d), which reects the fact that the 6s 2 non-bonded electron pairs of Bi 3+ are stereochemically active.The Tb atom occupies the Wyckoff 2a position with local D 3d symmetry.It is surrounded by six oxygen atoms in an octahedral environment with six equal Tb-O bond distances of 2.255(6) Å, which are very close to those reported in K 3 TbB 6 O 12 [2.229(4)Å-2.348(5)]. 37Two B atoms are each connected to three oxygen atoms, forming a BO 3 planar triangle with bond lengths in the range of 1.372(

Fig. 1
Fig. 1 The unit cell of Ba 4 BiTbO(BO 3 ) 4 projected approximately along [100] (a), the sheets A and B viewed along [001] (b and c), and the local environment of each metal cation site (d).Ba1, Ba2, and Ba3: black, blue, and orange balls, respectively; BiO 7 : magenta hexagonal pyramids; TbO 6 : deep sky blue octahedra; B1O 3 and B2O 3 : cyan and green triangles, respectively.In plot (c), when the structure is viewed along [001], two B2O 3 triangles overlap, while two BiO 7 hexagonal pyramids share a corner to form a Bi 2 O 13 dimer.
a). † The XPS survey scan shows the respective photoemission peaks caused by the constituent elements (Ba, Bi, Tb, Eu, B, and O) and the Auger peaks of Ba and O (Ba MNN & O KLL).The appearance of the C 1s signal is due to the absorption of adventitious hydrocarbons on the sample surface during the sample handling for XPS measurements.The chemical composition and bonding information were further probed utilizing the detailed, high-resolution core-level XPS scans of Ba 3d, Bi 4f, Tb 3d, Eu 3d, B 1s, and O 1s, as illustrated in Fig. S5(b)-(g).†

3. 8 .
Emitting light color analysis In general, the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates are used to characterize the emission color of the phosphor material, and the Correlated Color Temperature (CCT) is used to study which type of light the

Fig. 9
Fig. 9 Temperature-dependent PL spectra of the Ba 4 BiTb 0.999 Eu 0.001 O(BO 3 ) 4 phosphor upon excitation at 349 nm (a) and 377 nm (b), respectively; the insets show the relative PL integral intensities of the phosphor.The linear fitting curves of ln(I 0 /I − 1) versus 1/kT for l ex = 349 nm (c) and l ex = 377 nm (d) are also depicted in the figure.

Fig. 10
Fig. 10 Temperature-dependent FIR (FIR = I 593 /I 543 , where I 593 and I 543 are the integrated emission intensities of Eu 3+ : 5 D 0 / 7 F 1 transition from 585 to 600 nm and Tb 3+ : 5 D 4 / 7 F 5 transition from 535 to 565 nm, respectively) of the Ba 4 BiTb 0.999 Eu 0.001 O(BO 3 ) 4 phosphor upon excitation at 377 nm.The fitted line for the experimental data is also provided in the figure.

Table 2
CIE chromaticity coordinates and CCT values of Ba 4 -BiTb 1−x Eu x O(BO 3 ) 4 (0 # x # 0.2) phosphors under the excitation of 349 and 377 nm can be partially replaced by other divalent cations, such as Pb 2+ and Sr2+.Additionally, a series of phosphors, Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 (0 # x # 1) and Ba 4 -BiYO(BO 3 ) 4 , were synthesized at 830 °C.Based on analyses of IR/ Raman and UV-vis diffuse reection spectra, the geometric deviation of the BO 3 group from the D 3h symmetry was veried, and the indirect and direct band gaps of Ba 4 BiLnO(BO 3 ) 4 (Ln = Y, Tb, Eu) were determined.Upon 349 nm excitation, the PL spectra of Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 contain blue Bi 3+ : 3 P 1 / 1 S 0 , green Tb 3+ : 5 D 4 / 7 F 6,5 and orange Eu 3+ : 5 D 0 / 7 F 1,2,3,4 emissions.With an increase in Eu 3+ content (x), the emission intensities of Bi 3+ and Tb 3+ decline steadily, whereas that of Eu 3+ initially increases until x = 0.05, and then decreases.When l ex = 377 nm, only Tb 3+ and Eu 3+ emissions appear simultaneously, and the emission intensity of Tb 3+ decreases, while that of Eu 3+ increases along with increasing x.Thus, a singlecomponent near-white emission and a color-tunable emission from green to red can be realized in Ba 4 BiTb 1−x Eu x O(BO 3 ) 4 by manipulating the Tb 3+ /Eu 3+ ratio and adopting different excitation wavelengths.Moreover, the Eu 3+ /Tb 3+ uorescence intensity ratio (I 593 /I 543 ) for Ba 4 BiTb 0.999 Eu 0.001 O(BO 3 ) 4 upon 377 nm excitation can be linearly related to the temperature between 303 and 483 K, indicating the potential of the phosphor as a promising temperature sensor.These ndings indicate that exploring new borates based on element substitution is feasible.By doping rare-earth activators onto the newly prepared borate matrix, multifunctional uorescent materials can be obtained, which may nd potential use for w-LEDs and temperature sensor applications.This is a promising eld of research that is worth further studying.